1. Field of the Invention
The present invention relates to an optical emission spectrometry device in which the constituting atoms of a sample are evaporated to emit light through discharge and the optical intensity is measured for analyzing the elementary composition of the sample. More particularly, the present invention relates to an optical emission spectrometry device in which a high-current spark discharge is generated between a metal sample and a discharge electrode, and a plurality of elements is analyzed simultaneously in a short time.
2. Description of Related Art
A spark discharge is generated by an optical emission spectrometry device between a metal sample and a discharge electrode (a discharge gap). Through a high-current discharge, the atoms on the surface of the metal sample are evaporated, and the evaporated atoms are excited by discharge plasma. As the excited atoms emit light with a line spectrum inherent in the respective elements, the amount of the elements existing in the plasma is obtained by introducing the light into a spectrometer to measure the intensity of the light of a specific wavelength. Therefore, through simultaneous measurements on lights of a plurality of wavelengths, the amount of each element in the plasma is determined. Based on the resulting information, the elementary composition of the metal sample is derived.
In an optical emission spectrometry device in the prior art, a capacitor charged with several hundreds of volts is connected in advance between a metal sample and an electrode (a discharge gap), and discharge is commenced by an ignition circuit 12 (12a-12d). The ignition circuit may be connected in series to a discharge path formed by the discharge gap with a coil 13 and a capacitor 14, as shown in FIGS. 1A and 1C, or may be connected in parallel to the above discharge path, as shown in FIGS. 1B and 1D. Upon the beginning of the discharge, the energy stored in the capacitor 14 increases the discharge current rapidly, and creates a high-energy spark discharge between a metal sample 32 and a discharge electrode 31. In the mean time, the temperature on the surface of the metal sample 32 is significantly elevated, such that the atoms constituting the sample begin to be evaporated. The coil 13 is connected to limit the discharge current. Additionally, a recharge circuit 15 is also disposed for recharging the capacitor 14, which has lost energy due to the discharge.
The evaporated atoms are excited by electrons in the plasma. Afterward, when resuming a stable state, the excited atoms emit a light with a wavelength corresponding to the energy difference. As each element has its inherent energy levels, the wavelengths of the light also form an inherent line spectrum of the elements. The emitted light in the plasma is efficiently introduced to the spectrometer, and the optical intensity indicative of each of the plurality of elements is simultaneously measured. The optical intensity of each wavelength is not simply proportional to the elementary composition, but is roughly proportional to the amount of each element, the elementary composition can be determined by computing the relationship between the optical intensity and the amount of the elements in advance and converting the optical intensity into the amount of the elements.
In order to prevent changes in the surface condition of the sample during the analysis, an inert gas, etc., is usually filled the space between the metal sample 32 and the discharge electrode 31. As the metal sample and the discharge electrode 31 are arranged with a space, i.e. a discharge gap, of about several millimeters, the discharge may not start by applying a voltage of several hundreds of volts. Therefore, the ignition circuit 12 is disposed to generate a high voltage in the discharge gap to initiate the discharge. The ignition circuit 12 may utilize the voltage boosting function of a transformer, as shown in FIGS. 1A and 1B, or utilize the electromotive force generated by breaking the current, as shown in FIGS. 1C and 1D, and so on. For example, a voltage of several hundreds of volts is applied to the primary circuit in the transformer, such that a high voltage of several kV to tens of kV is generated on the secondary circuit and applied to the discharge gap. Alternatively, a voltage of tens of volts is continually applied to the primary circuit at a fixed period of time. When the current turns into several amperes to tens of amperes, voltage that is being applied to the primary circuit ceases, such that an induced electromotive force is generated and a high voltage of several kV to tens of kV is generated on the secondary circuit. However, the primary circuit and the secondary circuit are not necessarily separated and can be interconnected. Alternatively, a coil may be excited to generate an induced electromotive force.
If a high voltage is generated in the discharge gap 11 by the ignition circuit 12, a discharging occurs in the gap and the discharge current flows in the gap, while the voltage in the gap decreases rapidly. If the capacitor 14 charged with several hundreds of volts is connected to the discharge gap 11, energy is supplied by the capacitor 14 to produce high-current plasma. However, as the impedance of the plasma decreases with the increase of the current, an excessively high current will occur if the capacitor 14 is directly connected to the discharge gap, such that the plasma cannot be maintained for a fixed period of time. Therefore, a coil 13 is usually connected between the capacitor 14 and the discharge gap 11 for controlling the increasing rate of the current and maintaining an appropriate current value. Through appropriate selections of the capacitance and charging voltage for the capacitor 14 and the inductance for the coil 13, a proper current waveform of several tens of amperes to several hundreds of amperes may be selected. Alternatively, as shown in FIG. 2, a more complicated current waveform can be synthesized by preparing in advance several series circuits of capacitors 14 and the coils 13 and connecting them in a proper combination circuit based on the application. In FIG. 2, same or like components are designated with identical reference numbers as in FIGS. 1A˜1D. In the figure, an ignition circuit is depicted by reference numeral 22, the coils are depicted by the reference numerals 23a-23c, the capacitors are depicted by the reference numerals 24a-24c, the recharge circuits are depicted by the reference numerals 25a-25c, and the discharge path switch elements are depicted by the reference numerals 27a-27c. In alteration, the combination of mechanical elements such as relays or semiconductor elements such as metal oxide semiconductor field effect transistors (MOSFETs) or thyristors, can be used. In addition, all of the recharge circuits may share a common power supply.
As such, the current waveform may be selected through the combination of capacitors and coils. However, as the resulting current waveform is determined by the combination of capacitors and coils, the current value cannot be selected optionally. Moreover, as the discharge ends due to the loss of the energy charged to the capacitor, the time duration of the discharge cannot be controlled unrestrictedly.
Therefore, in order to control the discharge current at an arbitrary value, a driving circuit 16, for example, as shown in FIG. 3 is used. Initially, a capacitor with sufficiently increased capacitance, which continually supplies a current to the discharge gap, or a power supply 34 is prepared. After that, a switch element 33 for switching the connection is disposed to connect the coil 13 to the capacitor or the power supply, or directly connect the coil 13 to the discharge path (ground). The capacitor is provided with a circuit for recharging. By using the driving circuit 16 consisting of the capacitor or the power supply 34 and the switch element 33, the value of the discharge current can be arbitrarily controlled. Such an approach is usually referred to as a forward converter in the field of power supply circuits. When the current value is smaller than a target value, the current through the coil increases after the coil is connected to the capacitor or power supply. When the current value exceeds the target value, the current through the coil is sustained with the electromagnetic energy stored in the coil after the coil is grounded. As the switch element must have a high switching speed of a certain degree, semiconductors such as MOSFETs shown in FIGS. 4A and 4B are often used. In FIG. 4A, components 35 and 36 are MOSFETs, and component 37 is a MOSFET control circuit. However, a diode 38 may be used as the element grounded as shown in FIG. 4B for simplicity.
In the conventional optical emission spectrometry device, the current waveform may be controlled through a forward converter approach. However, as the current increases when the coil is connected to the power supply and the current decreases when the coil is disconnected from the power supply, the current waveform is formed like a saw-tooth shape. Therefore, the optical intensity also oscillates in a saw-tooth shape, which impairs the spectrometry precision or reproducibility. By shortening the interval between driving circuit switches, variation in the current value may be reduced; however, as the switching loss of the switch element in the driving circuit is further increased, the problem of reducing energy transfer efficiency may occur.